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FXUS21 KWNC 241840
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT JULY 24 2023
SYNOPSIS: A MAJOR HEAT WAVE IS FORECAST TO PERSIST ACROSS THE SOUTHERN
CONTIGUOUS U.S. (CONUS) DURING EARLY AUGUST AS MID-LEVEL HIGH PRESSURE BUILDS
OVER MUCH OF THE WESTERN AND SOUTHERN CONUS BRINGING CONTINUED RISKS FOR
EXCESSIVE HEAT. THE COMBINATION OF RECENT DRYNESS AND PREDICTED ABOVE-NORMAL
TEMPERATURES AND BELOW-NORMAL PRECIPITATION DURING WEEK-2 SUPPORTS RAPID ONSET
DROUGHT (ROD) RISK OVER PARTS OF SOUTHERN TEXAS AND THE INTERIOR PACIFIC
NORTHWEST. ROD RISK IS ALSO ELEVATED ACROSS NORTHWESTERN NORTH DAKOTA DUE TO
ANTECEDENT DRY CONDITIONS AND LITTLE PRECIPITATION FORECAST DURING WEEK-1.
HAZARDS
HIGH RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE SOUTHERN PLAINS, LOWER
MISSISSIPPI VALLEY AND CENTRAL GULF COAST, TUE-WED, AUG 1-2.
MODERATE RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE SOUTHERN PLAINS,
MISSISSIPPI, AND TENNESSEE VALLEYS, AND THE SOUTHEAST, TUE-FRI, AUG 1-4.
SLIGHT RISK OF EXCESSIVE HEAT FOR MUCH OF THE SOUTHERN CONUS EXCLUDING PORTIONS
OF THE SOUTHERN CALIFORNIA COAST, TUE-MON, AUG 1-7.
SLIGHT RISK OF EXCESSIVE HEAT PARTS OF THE INTERIOR NORTHWEST, WED-FRI, AUG 2-4.
RAPID ONSET DROUGHT RISK ACROSS PORTIONS OF SOUTHERN TEXAS AND INTO LOUISIANA,
INTERIOR PORTIONS OF THE PACIFIC NORTHWEST, AND NORTHWESTERN NORTH DAKOTA.
DETAILED SUMMARY
FOR THURSDAY JULY 27 - MONDAY JULY 31:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY AUGUST 01 - MONDAY AUGUST 07: THE FIRST WEEK OF AUGUST IS FORECAST
TO BEGIN WITH POSITIVE 500-HPA HEIGHT ANOMALIES FOR MUCH OF THE SOUTHERN AND
WESTERN CONUS, HOWEVER, THE STRENGTH OF THESE ANOMALIES LOOKS LIKELY TO WEAKEN
ACROSS THE SOUTHEAST BY THE MIDDLE TO END OF WEEK-2. THE POSITIVE 500-HPA
HEIGHT ANOMALIES AT THE ONSET OF THE PERIOD INDUCE A STRONG TEMPERATURE
RESPONSE.THE PROBABILISTIC EXTREMES TOOLS (PET) FROM THE GEFS AND ECMWF
INDICATE A 50 TO 70% CHANCE FOR TEMPERATURES TO EXCEED THE 85TH PERCENTILE.
THIS IS SUPPORTED BY RAW DYNAMICAL TOOLS AS WELL, SHOWING INCREASED CHANCES FOR
APPARENT TEMPERATURES TO 110 DEG F, LEADING TO A HOT AND HUMID AIR MASS.
THEREFORE, A HIGH RISK OF EXCESSIVE HEAT IS POSTED FOR PORTIONS OF THE SOUTHERN
PLAINS, LOWER-MISSISSIPPI VALLEY AND CENTRAL GULF COAST FOR AUG 1-2. ACROSS
OTHER PORTIONS OF THE SOUTHEAST AND SOUTH-CENTRAL PLAINS, THE ECMWF AND GEFS
PETS CONTINUE TO SHOW ENHANCED CHANCES (30-50%) FOR TEMPERATURES TO EXCEED THE
85TH PERCENTILE DURING THIS PERIOD. ABOVE-NORMAL SEA SURFACE TEMPERATURES ARE
ALSO FORECAST TO CONTINUE IN THE GULF OF MEXICO AND WESTERN ATLANTIC OCEAN THAT
MAY HELP TO INCREASE ATMOSPHERIC MOISTURE LEADING TO HIGHER APPARENT
TEMPERATURES. THEREFORE A BROADER MODERATE RISK OF EXCESSIVE HEAT IS POSTED FOR
AUG 1-4 FOR MUCH OF THE SOUTHEASTERN CONUS. IN THE SOUTHWEST, PROBABILITIES FOR
EXCESSIVE HEAT ARE GENERALLY REDUCED RELATIVE TO PREVIOUS FORECASTS, THEREFORE,
ONLY A SLIGHT RISK OF EXCESSIVE HEAT IS POSTED FOR THIS REGION. THE SLIGHT RISK
EXTENDS TO MUCH OF THE REST OF THE SOUTHERN CONUS, TO ENCOMPASS THE MODERATE
AND HIGH RISK HAZARDS, FOR ALL OF WEEK-2. A SLIGHT NORTHWARD EXPANSION OF THE
SLIGHT RISK WAS CONSIDERED INTO PARTS OF THE MIDWEST WHERE THE GEFS PET
CONTINUES TO SHOW ENHANCED CHANCES (>40%) OF TEMPERATURES EXCEEDING THE 85TH
CLIMATOLOGICAL PERCENTILE IN THIS REGION. HOWEVER, THIS APPEARS TO BE A MODEL
ARTIFACT LIKELY AS A RESPONSE TO LOW-SOIL MOISTURE AND THE HAZARD WAS NOT
EXTENDED NORTH OF THE ECMWF GUIDANCE. FINALLY, A SLIGHT RISK OF EXCESSIVE HEAT
IS POSTED FOR INTERIOR PORTIONS OF THE PACIFIC NORTHWEST WHERE PETS SHOW
ENHANCED CHANCES FOR TEMPERATURES TO REACH THE 85TH PERCENTILE AND 100 DEGF FOR
AUG 2-4 UNDER POSITIVE 500-HPA HEIGHT ANOMALIES.
IN THE NORTHEAST, NEGATIVE 500-HPA HEIGHT ANOMALIES HAVE BECOME REESTABLISHED
AND ARE FAVORED TO REMAIN THROUGHOUT WEEK-2 BRINGING REDUCED CHANCES FOR
EXCESSIVE HEAT INTO THE NORTHEAST AND NORTH-CENTRAL CONUS WITH NORTHERLY FLOW
BECOMING MORE ESTABLISHED. THE TROUGHING FEATURE IN THE NORTHEAST MAY BRING
RENEWED CONCERNS FOR HEAVY PRECIPITATION TO THE NORTHEAST WHERE THERE HAS BEEN
REPEATED FLOODING IN RECENT WEEKS. HOWEVER, CONFIDENCE IS NOT PARTICULARLY
STRONG IN THIS EVENT AT THIS TIME, AND NO CORRESPONDING HAZARD IS POSTED.
IN THE SOUTHWEST, MID-LEVEL HIGH PRESSURE AND A SURFACE LOW NEAR THE NORTHERN
GULF OF CALIFORNIA MAY HELP TO BRING ENHANCED PRECIPITATION TO PARTS OF THE
AREA. HOWEVER, PRECIPITATION PETS FROM THE GEFS, ECMWF, AND CANADIAN DO NOT
HAVE A PARTICULARLY STRONG SIGNAL FOR HEAVY PRECIPITATION IN THIS REGION AS OF
YET, WITH ONLY SMALL AREAS SEEING CHANCES FOR GREATER THAN 0.5 INCHES OF
PRECIPITATION DURING A THREE DAY PERIOD. THEREFORE, NO CORRESPONDING HAZARD IS
POSTED FOR PRECIPITATION.
DUE TO LITTLE TO NO RAINFALL FORECAST AND THE ONGOING EXCESSIVE HEAT CONDITIONS
FAVORED TO PERSIST ACROSS THE SOUTHERN TIER, THERE CONTINUES TO BE AN ELEVATED
RISK OF RAPID ONSET DROUGHT (ROD) DEVELOPMENT FOR PARTS OF THE SOUTHERN PLAINS
AND LOWER MISSISSIPPI VALLEY GIVEN THE INCREASED EVAPOTRANSPIRATION RATES.
ADDITIONAL ROD RISK AREAS ARE INDICATED ACROSS INTERIOR PORTIONS OF THE PACIFIC
NORTHWEST AND NORTHWESTERN NORTH DAKOTA DUE TO SOIL MOISTURES IN BOTH OF THESE
REGIONS BEING BELOW THE 30TH CLIMATOLOGICAL PERCENTILE AND WITH LITTLE
ADDITIONAL RAINFALL FORECAST DURING WEEK-1.
NO HAZARDS ARE POSTED FOR ALASKA DURING WEEK-2. ABOVE-NORMAL TEMPERATURES ARE
PREDICTED ACROSS MUCH OF THE STATE. NEAR-NORMAL PRECIPITATION IS FORECAST
ACROSS MUCH OF THE MAINLAND, WITH NEAR- TO BELOW-NORMAL PRECIPITATION FORECAST
ACROSS SOUTHEAST ALASKA AND THE SOUTH COAST OF THE MAINLAND. ABOVE-NORMAL
PRECIPITATION IS FAVORED ACROSS PARTS OF THE NORTH SLOPE.
FORECASTER: RYAN BOLT
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